The goal of this project is the development of new antineoplastic agents directed against tubulin, a protein critical for cell division and for the maintenance of cellular morphology. Detailed mechanistic and structure- activity studies were performed with many classes of compounds. Analogs of combretastatin A-4 were examined to determine important structure- activity relationships. The interaction with tubulin of the pentapeptide dolastatin 10, a noncompetitive inhibitor of vinca alkaloid and nucleotide binding, was defined with great precision based on properties of stereoisomers, analogs, and segments of the active molecule. Studies on interactions of radiolabeled dolastatin 10 with tubulin were initiated. Structure-activity and mechanistic studies were continued or initiated with analogs of colchicine, derivatives of 2-phenyl-4-quinolone, derivatives of 5,6-diphenylpyridazin-3-one, and derivatives of estradiol. Radiolabeled A-ring modified analogs of colchicine that react covalently with beta-tubulin were synthesized. We initiated studies with photoactive analogs of taxol which should ultimately prove useful in defining the taxol binding site on tubulin and provide insight into the drug's mode of action. The DTP drug screening data base was examined using the COMPARE program with known antimitotic compounds as seeds. This yielded multiple leads for new antimitotic agents. The most potent were the halichondrins, and structure-activity and mechanistic studies were performed with these complex molecules. Studies were continued with a new series of potent marine natural products, the spongistatins. Work was initiated to attempt to define differences in cellular response to different antimitotic drugs with equivalent activity against tubulin, and to understand differences among cell types to antimitotic agents as demonstrated by the differential cytotoxicity uncovered by COMPARE analysis.